1. Field of the Invention
The present invention relates to honeycomb structural bodies for supporting catalyst capable of purifying exhaust gas emitted from an internal combustion engine mounted to motor vehicles, etc.
2. Description of the Related Art
Various types of honeycomb structural bodies have been known and used, which support catalyst. The catalyst is in general capable of purifying exhaust gas emitted from an internal combustion engine mounted to a motor vehicle, etc. For example, patent document 1, Japanese patent laid open publication No. 2008-18370, and patent document 2, Japanese patent laid open publication No. H07-246341, have disclosed a honeycomb structural body having a plurality of cells. Each of the cells is surrounded by cell walls. The cell walls are arranged in a lattice-like shape in a cross section along a longitudinal direction of the honeycomb structural body. That is, the cell walls form the cells arranged in a lattice-like shape. Each of the cells has a square shape or a hexagonal shape, for example.
Such a honeycomb structural body is mounted to the inside of an exhaust gas pipe connected to an internal combustion engine. The exhaust gas having a high temperature emitted from the internal combustion engine flows in the exhaust gas pipe. A catalyst reaction between the exhaust gas and the catalyst supported by the honeycomb structural body is generated in the inside of the exhaust gas pipe when the exhaust gas is passing through the inside of the honeycomb structural body mounted to the exhaust gas pipe. The exhaust gas is purified by the catalyst reaction in the honeycomb structural body, and the purified exhaust gas is discharged to the outside of the exhaust gas pipe.
Recently, because the vehicle emissions control of reducing motor vehicle emissions, etc. is becoming stricter year by year in view of environmental protection, there is a strong demand to decrease carbon monoxide contained in exhaust gas emitted from an internal combustion engine and more improve a fuel efficiency of motor vehicles. The above recent demand increases an amount of noble metal which is used as catalyst in the honeycomb structural body. From the viewpoint of increasing the catalyst cost and difficulty in procurement of resources such as noble metal, there is a strong demand of decreasing the amount of noble metal used in the honeycomb structural body. Still further, there is also a strong demand for the honeycomb structural body to have an excellent capability of purifying exhaust gas emitted from the internal combustion engine.
For example, the patent document 1 shows a honeycomb structure body having a structure in which a boundary partition wall divides a cell formation section into a central section and an outer peripheral section in a cross section along a direction which is perpendicular to a longitudinal direction of the honeycomb structure body. The outer peripheral section has an opening area ratio which is larger than an opening area ratio of the central section. The opening area ratio indicates a ratio of a whole area of opening sections to a whole area of cell walls in a section (such as the central section and the outer peripheral section) on a cross section which is perpendicular to a longitudinal direction of a honeycomb structure body. This structure makes it possible for exhaust gas to easily flow in the outer peripheral section when compared with in the central section because the outer peripheral section has a large opening area ratio when compared with the opening area ratio of the central section although the outer peripheral section is lower in a gas flowing rate than the central section. This also makes it possible to improve the exhaust gas purification performance of the honeycomb structure body.
However, the honeycomb structure body disclosed in the patent document 1 has imperfectly shaped cells in addition to normal cells (or perfectly shaped cells). The normal cell has a perfect shape surrounded by the cell walls only. On the other hand, one or more sides of the imperfectly shaped cells are surrounded by the boundary partition wall, i.e. each of the imperfectly shaped cells is in contact directly with the boundary partition wall. If the honeycomb structure body is produced by using an extrusion die, there is a possibility of deforming and breaking the cell walls and the boundary partition wall which is in contact directly with the imperfectly shaped cells. Further, there is a possibility that the cell walls near the imperfectly shaped cells are affected by the deformation of the cell walls and the boundary partition wall.
In addition, a catalyst application process is performed after the production of the honeycomb structure body. In the catalyst application process, a catalyst slurry containing catalyst such as noble metal is fed to the inside of the honeycomb structure body in order to apply catalyst on the inner walls of the cells. At this time, when an imperfectly shaped cell having a small-sized opening area is present in the honeycomb structure body, the imperfectly shaped cell is clogged by the catalyst during the catalyst application process. This decreases the gas flowing rate and increases a pressure loss of the honeycomb structure body. Still further, because the catalyst such as noble metal is supported on the catalyst clogged cells where exhaust gas does not flow easily, an amount of wasted catalyst increases.
In order to solve the problem previously described, there is the technique disclosed in the patent document 2 in which the overall imperfectly shaped cells, which are in contact directly with the boundary partition wall, are plugged or closed by the same material with which the cell walls are formed.
FIG. 8 is a schematic view showing a part of an extrusion metal die 8 to be used when a honeycomb structural body is produced. That is, FIG. 8 is a view showing a schematic structure of the extrusion metal die 8. In FIG. 8, a feeding direction is designated by the arrows. FIG. 8 shows a part of the extrusion metal die 8 comprised of feeding holes 81 and slit grooves 82. Raw material is fed through the feeding holes 81. The feeding holes 18 are communicated with the slit grooves 82. The cell walls and the boundary partition wall are formed by the slit grooves 82. Blocks 89 are removed from the extrusion metal die 8 in order to form a honeycomb structural body in which the overall imperfectly shaped cells are plugged or closed.
By the way, when the extrusion metal die 8 has a structure in which the overall imperfectly shaped cells are plugged or closed, and uses the extrusion metal die 8 during the process of producing the honeycomb structure body, a difference in feeding speed and feeding amount of raw material is generated between the parts corresponding to the cell walls forming the normal cells (or the perfectly shaped cells) and the parts corresponding to the cell walls forming the imperfectly shaped cells. That is, because a feeding resistance of raw material in the parts in the extrusion metal die (which form the imperfectly shaped cells to be plugged) is decreased, this increases a feeding speed and an amount of raw material at these parts in the extrusion metal die (which correspond to the imperfectly shaped cells). Further, this generates shape deformation and defects of the cell walls which are adjacent to the imperfectly shaped cells because the raw material is provided to the imperfectly shaped cells caused by the increased feeding speed and amount thereof. As a result, there is a possibility of decreasing the whole strength of the produced honeycomb structural body. Furthermore, when the opening section of all of the imperfectly shaped cells in the produced honeycomb structural body are completely plugged or closed, a pressure loss of the produced honeycomb structural body is increased and a specific surface area of the cell walls in the honeycomb structural body is decreased. As a result, there is a possibility of decreasing a fuel efficiency of an internal combustion engine, an output performance of the internal combustion engine, deteriorating a performance of purifying exhaust gas by the decreased specific surface area of the cell walls, where the specific surface area is necessary to perform a correct catalyst reaction between exhaust gas and catalyst.